1. Field of the Invention
This invention relates to a lead frame and particularly to one used to package a semiconductor integrated circuit (IC) chip or chips.
2. Description of the Prior Art
To package IC chips a lead frame 1 as shown in FIG. 1 has hitherto been used. A blank metal sheet made of copper is etched or otherwise cut to form the lead frame 1 as an integral unit which is composed of an outer frame 2 and inner lead frame portions 8 where leads 7 radiate in all directions. On each side of a lead frame portion 8, outer leads 6 are connected through narrow straight strips 9 and 10. These strips join the outer frame 2 at the corners of the lead frame portion 8. Leads extending from the corners of the mount pad 11 are for mounting an IC chip (not shown) thereon and join the outer frame 2 at the corners of the lead frame portion 8.
Accordingly, the lead frame 1 of FIG. 1 has the lead frame portion 8 directly connected to the outer frame 2 at each corner. It is noted that after IC packaging, the area encircled by a profile 3 is sealed in resin; A hole 4 is used position the lead frame 1.
If the lead frame 1 is passed across a process step that is involved in such as the chip bonding step, there will appear temperature differences between the lead frame portion 8 and outer frame 2, and result in deformation of leads 6 and 7. This impairs the position accuracy. As the resin cast in the region 3 shrinks, leads 6 are forcibly pulled on all the four sides of the lead frame region 8 because of different thermal expansion coefficients of the resin and lead frame, which results in deformations at points 1, 2, and 6. Such deformations not only make it difficult to convey the lead frame smoothly to subsequent steps of the production line but also cause slipping of leads 7 in the cast resin leading to poor moistureproof performance and less reliability of the product.